1. Field of the Invention
The present invention relates to a photographic fish eye lens system having a field angle of 180.degree. and more particularly to an improved fish eye lens system having an aperture ratio of F/2.8.
2. Description of the Prior Art
In the photography field, the advantages of extremely wide angle lens systems and more particularly, fish eye lens systems are well known. There is a continual desire in the camera industry to provide extremely lightweight and compact lens systems for use with conventional camera bodies such as the single lens reflex camera. Unfortunately, these demands are difficult to accomplish while maintaining 180.degree. field angle. As the fish eye lens system is made more compact, the light rays passing through the system are subjected to extreme bending which creates considerable problems in compensating for lateral chromatic aberration, astigmatism and curvature of the field. Another problem with a 35 mm SLR camera is the positioning of the reflecting mirror, which requires the fish eye lens system to have a relatively long back focal length. A suggested solution to some of these problems can be found in the inventor's earlier work disclosed in U.S. Pat. No. 4,009,943 where an aperture ratio of F/4 was achieved for use with a compact single lens reflex camera.
Generally, the solution of the problems of field curvature, astigmatism and lateral chromatic aberrations require the diameter of the front lens element and the total length of the lens system along the optical axis to be increased. Regretfully, this is directly at odds with a compact format size.
Numerous examples of fish eye lens systems are disclosed in the patent literature.
The Ogura U.S. Pat. No. 3,597,049 provides one example of solving the problems associated with a fish eye lens system by controlling the Abbe values of the lens elements. The Japanese Patent Nos. 47-23576; 47-35019 and 39-18718 are additional examples of fish eye lens systems. Other examples of prior art patents can be found in the Yamashita et al U.S. Pat. No. 3,870,400; Zimmerman et al U.S. Pat. No. 3,132,199; Mukai U.S. Pat. No. 2,969,713; Nakagawa U.S. Pat. No. 3,741,630; Shimizu U.S. Pat. No. 3,734,600 and Kazamaki et al U.S. Pat. No. 3,515,462.
The prior art is still seeking to provide a compact improved fish eye lens system of a relatively high speed that is both photographically acceptable and which can be manufactured relatively inexpensively.